1. Field of the Invention
The present invention relates to a ultraviolet electroluminescent element and a laser luminescent element capable of emitting in ultraviolet range.
2. Description of the Related Art
Electroluminesce (hereinafter referred to as xe2x80x9cELxe2x80x9d) which is generated by application of strong electric field to a fluorescence body has two types, one is a current injection type EL such as a light emitting diode and another is a voltage excitation type EL. As the voltage excitation type EL, there are known a dispersion powder type EL panel in which material that is obtained by dispersing fine fluorescence powder into a synthetic resin or a glass powder is disposed between a transparent electrode and a back electrode, and a double insulation film EL panel in which a film-shaped fluorescent body emitting layer made by vacuum evaporation or spattering method is completely covered by a dielectric insulating layer and is disposed between a transparent electrode and a back electrode. The emitting color of a voltage excitation type EL element varies with a fluorescent material. A fluorescent material obtained by adding 0.3 to 0.5 weight percents of manganese to zinc sulfide (ZnS:Mn) provides yellow-orange color; SrS:Ce blue color, CaS:Ce or CaS:Er green color; and CaS:Eu red color. Besides, fluorescent material ZnS:TmF3 provides blue color; ZnS:TbF3 green color; and ZnS:SmF3 orange-red color.
Further, in recent years, an injection type EL element with two hole transporting layers and an emitting layer has been highlighted. FIG. 11 shows a cross section of the two-layer EL element, in which on a transparent electrode (ITO) 92, that is formed on a glass base plate 91, are mounted a hole transporting layer 93 and an emitting layer 94, and an upper electrode 95 is further formed thereon. Aromatic diamine derivative or polymethyl phenylsilane is used for the hole transporting layer 93, and 8-hydroxy quinoline aluminum (Alq3), that is an emitting metal complex, is used for the emitting layer 94. The upper electrode 95 is an electrode in which Mn and Ag are laminated. The hole transporting layer 93 functions to transport holes and to block electrons, which prevents the electrons from being transported to the electrode without rebonding with the holes.
When the EL element shown in FIG. 11 is operated in continuous direct current mode under the condition of positive ITO and forward bias, a bright green emission color is generated. FIG. 12 shows the emitting spectrum of the EL element and Alq3. In this figure, a solid line shows the spectrum of the EL element and a dotted line shows the spectrum of Alq3. The spectrum of the EL element coincides with that of Alq3, so that the EL is from Alq3 (Polymer Preprints, Japan, 40(3), 1071(1991); Applied Physics Letter, 59(21), 2760).
In a paper xe2x80x9cPolymer Preprintsxe2x80x9d (Polymer Preprints, Japan, 44(3), 325 (1995)) is stated that polysilane with oxygen bridge formation structure emits in an electrical field. According to the paper, polymethyl phenylsilane (PMPS) is painted on an ITO base plate and is bridged under heat, and then single-layer EL element with ITO/bridged PMPS/Al structure to which Al is evaporated emits in the electrical field with emission energy center of 1.8 eV. It is stated, in this paper, that normal polysilane without oxygen bridge formation structure does not emit.
In optical recording to record data to a recording media through a light ray, recording density can be improved as recording wave length becomes shorter, therefore, it is advantageous to utilize a small light source which emits in the ultraviolet range. Further, since many fluorescent pigments emit fluorescence while absorbing ultraviolet light, if an ultraviolet plane light source is realized, it becomes possible to constitute a display panel by applying fluorescent pigment thereon. In an optical system utilizing ultraviolet light, if the emitting wavelength purity of an ultraviolet light source should be too high, it would be easy to design diffraction gratings and mirrors which are adopted to the system. As described above, latent demand for an ultraviolet light source which is easily handled is strong.
An EL element with emission spectrum in the visible range is already known as described above, however, no EL element emitting in the ultraviolet range is known. Besides, a conventional EL element is, as typically illustrated in FIG. 12, produces a broad emission spectrum.
It is therefore an object of the present invention to provide an EL element capable of emitting ultraviolet light with high wavelength purity.
It is another object of the present invention to provide a solid laser luminescent element capable of emitting in a range including the ultraviolet range.
In the present invention, a thin film made from a polymer or oligomer which is formed by directly bonding elements which are selected from Si, Ge, Sn, and Pb (those elements may be the same as each other or may be different from each other) as an emission layer of an EL element or a laser luminescent element is utilized to accomplish the above-mentioned objects. In order to cause the EL element or the laser luminescent element to efficiently emit, the polymer or oligomer needs to have six or more atoms in a main chain structure.
That is, the EL element or the laser luminescent element according to the present invention is characterized in that a thin film made from a polymer or oligomer which is formed by directly bonding elements selected from Si, Ge, Sn, and Pb (those elements may be the same as each other or may be different from each other) is disposed between two electrodes, and at least one of the electrodes is transparent. In case of the laser luminescent element, however, it is not always necessary that one of the electrodes be transparent.
As the polymer or oligomer in which elements selected from Si, Ge, Sn, and Pb are directly bonded (those elements may be the same as each other or may be different from each other), there may be used as described below chemical formula 1, polymer or oligomer in which elements selected from Si, Ge, Sn, and Pb are the same as each other, and the elements are directly bonded, or as described below chemical formula 2, polymer or oligomer in which elements selected from Si, Ge, Sn, and Pb are different from each other, and the elements are directly bonded. 
Here, M represents Si, Ge, Sn, or Pb, and R1 and R2 represent substituents of the aforementioned elements, and both of them may be the same as each other or different from each other. Alkyl group, allyl group, phenoxy group, alkoxyl group, alkylamino group, alkylthio group, alcoholic hydroxy group or the like may be selected as R1 and R2, however, they are not limited to the above-mentioned groups. 
Here, M1 and M2 represent Si, Ge, Sn, or Pb, and R3, R4, R5, and R6 represent substituents of the aforementioned elements, and may be the same as each other or different from each other. Alkyl group, allyl group, phenoxy group, alkoxyl group, alkylamino group, alkylthio group, alcoholic hydroxy group or the like may be selected as R2, R3, R4 and R6, however, they are not limited to the above-mentioned groups.
Polymers consisting of four kinds of 14th group elements Si, Ge, Sn, and Pb have basically the same physical properties, so that it is possible to obtain an EL element and a laser luminescent element with emission spectrum in the ultraviolet range from polymer or oligomer in which the above-mentioned elements are exchanged with each other. Since this kind of EL element has considerably narrow emission band, it is possible to produce EL elements and laser luminescent elements with different emission wave lengths by changing the kinds of 4th group elements or a sequence of element arrangements.
It is known that the photoelectronic property of a polymer or oligomer which is formed by directly bonding elements selected from Si, Ge, Sn, and Pb (those elements may be the slame as each other or may be different from each other) strongly depends on the structure of a main chain, and it is possible to control, more or less, the main chain structure through a substituent. Therefore, the selection of the substitu ent permits the property as an EL element or a laser luminescent element to be changed. From this point of view, it is possible to use as an emission layer for example, a thin film made from polymer or oligomer in which the main chain structur e is structurally controlled, that is comformationally controlled so as to be suitable for emission condition as indicated by chemical formulas 3 and 4. A polymer or oligomer as in chemical formula 3 solidly draws a spiral, and the conformation thereof is liable to be relatively fixed under room temperature. A polymer or oligomer as in chemical formula 4 corresponds to those in which neighboring R1 or R2, or both of them, constitutes an alkyl group in combination with each other in the chemical formula 1, and are characterized in that the conformation thereof is liable to be relatively fixed under room temperature also. In order to increase the mechanical strength of polymer or oligomer, it may be possible to adopt a structure in which reinforcing substituents such as alkyl chains are added for bridging at some places. 
Here, M represents Si, Ge, Sn, or Pb, and Ro represents an optically active substituent. A 2-methyl butyl group may be used as an optically active substituent. R7 represents a substituent of the above-mentioned elements, and they may be the same with each other or may be different from each other. Alkyl group, allyl group, phenoxy group, alkoxyl group, alkylamino group, alkylthio group, alcoholic hydroxy group or the like may be selected as R7, however, it is not limited to the above-mentioned groups. 
Here, M represents Si, Ge, Sn, or Pb, and R8 and R9 represent substituents of the above-mentioned elements and they may be the same as each other or different from each other. Alkyl group, allyl group, phenoxy group, alkoxyl group, alkylamino group, alkylthio group, alcoholic hydroxy group or, like may be selected as R8 and R9, however, they are not limited to the above-mentioned groups.
Further, as indicated in the next chemical formula 5, it is possible to use a polymer or oligomer in which the same kind of 4th group elements with only one substituent are directly bonded. 
Here, M represents Si, Ge, Sn, or Pb, and R represents substituents of the above-mentioned elements and they may be the same as each other or different from each other. Alkyl group, allyl group, phenoxy group, alkoxyl group, alkylamino group, alkylthio group, alcoholic hydroxy group or the like may be selected as R, however, it is not limited to the above-mentioned groups.
Conventional methods such as spin coating, vacuum evaporation, optical CVD, and MBE (molecular beam epitaxy) may be utilized to form the emission layer film. In the case where a n emission layer is directly formed on the silica glass base plate by the CVD method, it is advantageous to use a silica glass base plate with a silane-treated surface.
With a polymer or oligomer which is used as an emission layer in the present invention, it is possible to control the emission wave length by changing the number of atoms of Si, Ge, Sn, and Pb which constitutes the main structure. Generally, as the chain length of the polymer or oligomer becomes longer, the peak wavelength shifts to the long wavelength side.
A polysilane layer of a conventional EL element only functions as a hole transporting layer. On the other hand, in the present invention, the emission of polysilane itself is utilized, which provides an ultraviolet electroluminescent element which has not been developed before.